Early studiers of cults were surprised to discover than when cults receive a major shock—a prophecy fails to come true, a moral flaw of the founder is revealed—they often come back stronger than before, with increased belief and fanaticism. The Jehovah's Witnesses placed Armageddon in 1975, based on Biblical calculations; 1975 has come and passed. The Unarian cult, still going strong today, survived the nonappearance of an intergalactic spacefleet on September 27, 1975. (The Wikipedia article on Unarianism mentions a failed prophecy in 2001, but makes no mention of the earlier failure in 1975, interestingly enough.)

Why would a group belief become stronger after encountering crushing counterevidence?